Among ventilation systems, air conditioners and air cleaners, sometimes a centrifugal fan such as a turbofan or a diagonal-flow fan of the type that sucks in gas from a rotating shaft direction and blows out the gas in a direction intersecting the rotating shaft is used. The impeller configuring the centrifugal fan is mainly configured by a resin-made end plate (main plate) that is rotated around the rotating shaft by a drive mechanism such as a motor, plural resin-made blades that are annularly disposed around the rotating shaft and integrally molded with the end plate, and a resin-made end ring (side plate) that is disposed such that it sandwiches the plural blades between itself and the end plate in the shaft direction and is fixed to the plural blades. In order to reduce the weight of the impeller, sometimes the blades are made hollow by disposing a space inside the blades when the plural blades are integrally molded with the end ring (e.g., see Japanese Patent Publication No. 8-159091 and Japanese Utility Model Application Publication No. 4-116699).
On the other hand, in order to improve the blowing performance and noise performance, sometimes the blades are given shapes that extend in the shaft direction while twisting between the end plate and the end ring (called “three-dimensional blades” below). However, when blades comprising three-dimensional blades are integrally molded with the end plate as hollowed blades (called “hollow blades” below), only a small space can be formed inside the blades due to restrictions on the direction in which they are removed from the mold, and the weight of the impeller cannot be sufficiently reduced.
Also, the plural blades are welded to the end ring by ultrasonic welding, and when the blades are made into hollow blades as described above, sometimes it becomes difficult to weld the blades to the end ring by ultrasonic welding because the blades become thin and their strength is reduced.